1. Field of the Invention
The present invention relates to a lens holding device and to a lens system that is equipped with the device. Specifically, the present invention relates to a lightweight, low-cost lens holding device and to a triplet lens system that employs the device, both of which are utilized on a portion of an optical system.
2. Background of the Related Art
In recent years, the number of optical systems that employ a plastic material in part of the lens system has increased. Along with this increase, several lens holding components have been proposed that make use of the advantageous characteristics of plastic. In particular, triplet lenses that employ one glass lens and two plastic lenses have come into wide use as lenses for inexpensive compact cameras.
An example of a holding device for this type of triplet lens optical system is a device introduced in Japanese Laid Open Patent Application Sho 63-141011.
With this conventional lens holding device, the diaphragm is positioned in between the second lens, which is a plastic lens, and the third lens, which is also a plastic lens. The diaphragm is in contact with both lenses. The second lens and the first lens are each inserted into the lens barrel as separate components and are fixed in place by a retaining ring.
The above-mentioned lens holding device is structurally simpler than earlier lens holding devices. However, it is structurally complicated to insert the plastic lenses holding the diaphragm into the lens barrel, and sufficient cost reduction is not achieved.
On a conventional optical system, in order to effectively shelter the lens system from light rays that cause "ghosts" or flares, it is common to position at least two light ray protecting devices in the vicinity of the incident surface and in the vicinity of the exit surface.
As shown by the example in the above-mentioned Japanese Laid Open Patent Application Sho 63-141011, a thin plate with an aperture can be inserted between lens L2 and lens L3 in the position shown by reference numeral 105 as the second light ray protecting device that is to be positioned in the vicinity of the exit surface, forming a fixed diaphragm.
However, with the method in which this type of fixed diaphragm formed from a thin plate is inserted between lens L2 and lens L3, the lens L2 can easily become eccentric with slight curvings or bendings of the thin plate. Thus, there is a danger of reducing the optical efficiency.
In addition, from the standpoint of optics, the second light ray protecting device should be as close as possible to the exit surface. Therefore, it is not desirable to have a structure such that a fixed diaphragm formed from a thin plate is inserted between lens L2 and lens L3.
A structure is also conceivable in which a fixed diaphragm is bonded in the position of the light ray exit surface 109. However, in this structure, the process of bonding the fixed diaphragm must be added. The addition of this process drastically reduces the manufacturing efficiency. Furthermore, an increase in the thickness of the lens barrel 4 with respect to the effective diameter of the lens L3 to ensure a sufficient bonding area cannot be avoided.
On a conventional triplet lens, the third lens L3 is commonly formed by a methacryl resin. This choice of material has two disadvantages.
The first disadvantage is that there are large variations in shape due to changes in humidity, and the optical efficiency does not remain constant throughout environmental changes. Another disadvantage is that, since the dispersion of the third lens L3 is a low dispersion when compared with the glass material used in the first lens L1, the off-axis chromatic aberrations become over-corrected when compared with the axial chromatic aberrations on the axis, resulting in a reduction in the off-axis optical efficiency.